The present invention relates to a radio communication system having a random access channel for the transmission of data from a secondary station to a primary station, and further relates to primary and secondary stations for use in such a system and to a method of operating such a system. While the present specification describes a system with particular reference to the emerging Universal Mobile Telecommunication System (UMTS), it is to be understood that the techniques described are equally applicable to use in other mobile radio systems. In this specification the term random access channel refers to the logical channel on which random access transmissions take place, which would typically consist of a number of distinct physical channels.
A random access channel is a normal component of a radio communication system, enabling a Mobile Station (MS) to send short messages to a Base Station (BS). Applications include signalling to the BS when the MS is turned on, sending a packet of data to the BS when the MS is not engaged in a call, and requesting the BS to allocate a resource for the MS to use.
In a system where mobile stations often have a requirement to send packets of data to the BS when not actually engaged in a call it is advantageous to provide a random access packet channel with similar characteristics to a standard random access channel but intended for the transmission of small and medium sized packets from a MS to the BS. Because of the random access nature of the channel, it is possible that two (or more) mobile stations attempt to send data packets simultaneously, leading to a high probability of data loss. The result of this collision is significant delay, as well as increased interference and loss of capacity.
An embodiment of a random access packet channel, developed for UMTS, employs a contention resolution method when a MS attempts to gain access to the packet channel. However, even using this method there is still a significant chance of unresolved collisions leading to significant reduction in system capacity.
An object of the present invention is to provide improved contention resolution.
According to a first aspect of the present invention there is provided a radio communication system comprising a primary station and a plurality of secondary stations and having a random access channel for the transmission of data from a secondary station to the primary station, the primary and secondary stations having means for engaging in an access phase, comprising the secondary station transmitting an access preamble encoded with a first signature and the primary station transmitting an access acknowledgement, the primary and secondary stations also having means for engaging in a first contention resolution phase on completion of the access phase, characterised in that the primary and secondary stations have means for engaging in a second contention resolution phase for at least some choices of the first signature, the primary station having means for granting the secondary station access to the random access channel on completion of the final contention resolution phase and the secondary station having means for transmitting data on the random access channel when it is granted access.
According to a second aspect of the present invention there is provided a primary station for use in a radio communication system having a random access channel for the transmission of data from a secondary station to the primary station, the primary station having means for engaging in an access phase, comprising the secondary station transmitting an access preamble encoded with a first signature and the primary station transmitting an access acknowledgement, and also having means for engaging in a first contention resolution phase on completion of the access phase, characterised in that means are provided for engaging in a second contention resolution phase for at least some choices of the first signature, and means are provided for granting the secondary station access to the random access channel on completion of the final contention resolution phase.
According to a third aspect of the present invention there is provided a secondary station for use in a radio communication system having a random access channel for the transmission of data to a primary station, the secondary station having means for engaging in an access phase, comprising the secondary station transmitting an access preamble encoded with a first signature and the primary station transmitting an access acknowledgement, the secondary station also having means for engaging in a first contention resolution phase on completion of the access phase, characterised in that means are provided for engaging in a second contention resolution phase for at least some choices of the first signature, the primary station having means for granting the secondary station access to the random access channel on completion of the final contention resolution phases and means are provided for transmitting data on the random access channel when access is granted by the primary station on completion of the contention resolution phase.
According to a fourth aspect of the present invention there is provided a method of operating a radio communication system having a random access channel for the transmission of data from a secondary station to a primary station, the method comprising the primary and secondary stations engaging in an access phase, comprising the secondary station transmitting an access preamble encoded with a first signature and the primary station transmitting an access acknowledgement, the method further comprising the primary and secondary stations engaging in a first contention resolution phase on completion of the access phase, characterised by the primary and secondary stations engaging in a second contention resolution phase for at least some choices of the first signature, the primary station granting the secondary station access to the random access channel on completion of the final contention resolution phase and the secondary station transmitting data on the random access channel when it is granted access.